CMOS image sensor capable of increasing punch-through voltage and charge integration of photodiode, and method for forming the same

ABSTRACT

A CMOS image sensor capable of increasing the punch-through voltage and the charge integration of a photodiode, and a method for forming the same. The punch-through voltage of a transfer transistor is increased, and the potential barrier is heightened between the photodiode and the floating diffusion region during the turn-off of the transfer transistor so as to increase the charge accumulation amount of the photodiode, while the photodiode is formed without resorting to a self-aligned ion-implantation. A p-type impurity region is formed under the gate electrode of the transfer transistor and within the semiconductor substrate, and the process can proceed without being limited by the self-alignment. Further, the p-type impurity region heightens the potential barrier between the photodiode and the floating diffusion region, i.e., the potential difference between the two regions is increased and, therefore, the charge accumulation amount is increased in the photodiode.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for fabricating a CMOSimage sensor. Particularly, the present invention relates to a CMOSimage sensor capable of increasing the punch-through voltage and thecharge integration of a photodiode, and a method for forming the same.

BACKGROUND OF THE INVENTION

[0002] The CMOS image sensor is a device for converting optical imagesto electrical signals. That is, it responds to the visible light, andthe signal electrons thus formed are converted to voltages. Then thevoltages are subjected to a signal processing to reconvert the voltagesto image information.

[0003] Used for recalling images, the CMOS image sensor is widelyapplied to cameras, medical equipment, monitoring devices, variousindustrial apparatuses for locating and sensing, toys, and the like.Such devices are driven with a low voltage, and a single chip issufficient in most cases. Accordingly, the field of application forthese sensors is being gradually expanded.

[0004] In the CMOS image sensor, there are formed as many MOStransistors as the number of pixels, and a switching method is adoptedin which the outputs are checked one by one. In the CMOS image sensor,compared with the conventionally and widely used CCD image sensor, thedriving method, is simple, and diversified scanning methods can berealized while the signal processing circuit can be packed into a singlechip. Accordingly, not only can the product be miniaturized, but also ahigh compatibility can be achieved with the result being reducedmanufacturing cost and power consumption.

[0005]FIG. 1 is a circuit diagram of a unit pixel of a conventional CMOSimage sensor that includes four transistors and two capacitors. That is,there is a photodiode as a photo-sensing means, and four NMOStransistors.

[0006] The four NMOS transistors serve the following roles. A transfertransistor Tx carries the photoelectric charges of the photodiode PD toa floating diffusion region. A reset transistor Rx discharges theelectric charges from the floating diffusion region so as to make itpossible to detect the signals. A drive transistor Dx serves as a sourcefollower, and a select transistor Sx carries out the switching andaddressing.

[0007] In the drawing, reference code Cf indicates the capacitance ofthe floating diffusion region, and Cp indicates the capacitance of thephotodiode. The image sensor thus constituted operates in the followingmanner. First, the reset transistor Rx, the transfer transistor Tx andthe select transistor Sx are turned on, thereby resetting the unitpixel.

[0008] Under this condition, the photodiode PD begins to be depleted,and a carrier-charging occurs in the capacitance Cp, while thecapacitance Cf of the floating diffusion region is charge-accumulated upto the supply voltage Vdd. Then the transfer transistor Tx is turnedoff, and the select transistor Sx remains turned on, while the resettransistor Rx is turned off.

[0009] In this operation, an output voltage V1 is read from a unit pixeloutput terminal (Out) and stored in a buffer. Then the transfertransistor Tx is turned on, and thus, the carriers of the capacitanceCp, which have been affected by the intensities of the visible light,are moved to the capacitance Cf. An output voltage V2 is read from theterminal (Out), and the difference between the two voltages (V1-V2) isconverted from an analog data to a digital data, thereby completing anoperational cycle of the unit pixel.

[0010] Now referring to FIGS. 2A to 2C, the conventional fabricationprocesses will be described for the transfer transistor, the photodiodeand the floating diffusion region of the CMOS image sensor.

[0011] First, as shown in FIG. 2A, a gate insulating film 22 and a gateelectrode 23 are formed on a p-type semiconductor substrate 20 on whicha device isolating film 21 has been formed. Further, at one end of thegate electrode 23 and within the semiconductor substrate 20, there isformed an n type impurity region 24 that will form a photodiode PD.

[0012] Then as shown in FIG. 2B, an insulating spacer 25 is formed on aside wall of the gate electrode 23, and a p-type impurity region 26 isformed on the n-type impurity region 24, thereby completing theformation of the photodiode.

[0013] Thereafter, in order to form a floating diffusion region, aphotoresist pattern PR is formed by using an ion-implantation mask, andan n-type dopant is ion-implanted, thereby forming a floating diffusionregion 27. Then as shown in FIG. 2C, the photoresist pattern PR isremoved.

[0014] In the above described conventional CMOS image sensor fabricatingprocess, a self-aligning ion-implantation is carried out to formingn-type impurity region of the photodiode by utilizing the gate electrode23 having the insulating spacer 25.

[0015] Further, the transfer transistor for moving the charges from thephotodiode to the floating diffusion region consists of a native NMOStransistor in which the threshold voltage has been adjusted to less than0 V so as to prevent a voltage drop. The lower portion of the transfertransistor Tx simply consists of a p-type epitaxial layer.

[0016] As the size of the chip is reduced, the channel length of thetransfer transistor is shortened. Therefore, a punch-through occurs ateven a low voltage so as to cause leakage.

[0017] Further, when the transfer transistor is turned off, thepotential barrier is lowered between the photodiode and the floatingdiffusion region and, therefore, the charge accumulation amount isdecreased in the photodiode during the charge integration. Further, ifthe self-alignment is not well designed when forming the photodioderegion, a process variation may occur.

SUMMARY OF THE INVENTION

[0018] The present invention is intended to overcome the above describeddisadvantages of the conventional technique.

[0019] Therefore it is an object of the present invention to provide aCMOS image sensor and a fabricating method therefor, in which thepunch-through voltage of a transfer transistor is increased, thepotential barrier is heightened between the photodiode and the floatingdiffusion region during the turn-off of the transfer transistor so as toincrease the charge accumulation amount of the photodiode, and thephotodiode is formed without resorting to a self-aligningion-implantation.

[0020] In achieving the above object, the CMOS image sensor having atransfer transistor for transferring charges from a photodiode to afloating diffusion region according to the present invention includes asemiconductor substrate; a gate electrode of the transfer transistor,the gate electrode being formed on the semiconductor substrate; aphotodiode including a first conduction type first impurity region and asecond conduction type second impurity region, the two regions beingformed at one end of the gate electrode and within the semiconductorsubstrate; a floating diffusion region including a second conductiontype third impurity region, the third impurity region being formed atanother end of the gate electrode and within the semiconductorsubstrate; and a first conduction type fourth impurity region formedunder the gate electrode and within the semiconductor substrate, andisolated from the photodiode and the floating diffusion region.

[0021] In another aspect, the present invention includes a method forforming a CMOS image sensor with a transfer transistor included thereinfor transferring the charges from a photodiode to a floating diffusionregion, the method including steps of forming a first conduction typefirst impurity region in a photodiode-forming region and within asemiconductor substrate; forming a second conduction type secondimpurity region in a transfer transistor region and within thesemiconductor substrate; forming a gate insulating film and a gateelectrode of the transfer transistor on the semiconductor substrate, aportion of the gate electrode being overlapped with the second impurityregion; forming a second conduction type third impurity region upon thefirst impurity region and within the semiconductor substrate; andforming a first conduction type floating diffusion region isolated fromthe photodiode region and across the gate electrode.

[0022] In still another aspect, the present invention includes a methodfor forming a CMOS image sensor with a transfer transistor includedtherein for transferring the charges from a photodiode to a floatingdiffusion region, the method including steps of forming a firstion-implantation mask for defining a photodiode-forming region in asemiconductor substrate; forming a first conduction type first impurityregion within the semiconductor substrate by carrying out anion-implantation; removing the first ion-implantation mask; forming asecond ion-implantation mask for defining a transfer transistor regionon the semiconductor substrate; forming a second conduction type secondimpurity region within the semiconductor substrate by carrying out anion-implantation; removing the second ion-implantation mask; forming agate insulating film and a gate electrode of the transfer transistor onthe semiconductor substrate, a portion of the gate electrode beingoverlapped with the second impurity region; forming a second conductiontype third impurity region upon the first impurity region and within thesemiconductor substrate; and forming a first conduction type floatingdiffusion region within the semiconductor substrate, isolated from thephotodiode region and across the gate electrode.

[0023] In the present invention, the CMOS image sensor is characterizedin that a p-type impurity region is formed under the gate electrode ofthe transfer transistor and within the semiconductor substrate.

[0024] Further, in the present invention, the fabricating process can beundertaken without being limited by self-alignment and, therefore, anyprocess variations can be inhibited. That is, in a design in which ann-type impurity region of a photodiode of 200 Kev or more has to beformed down to a deep depth, the channeling effect that is caused byself-alignment can be overcome.

[0025] Further, a p-type impurity region is formed under the gateelectrode of the transfer transistor and within the semiconductorsubstrate and, therefore, the potential barrier between the photodiodeand the floating diffusion region is heightened. That is, the potentialdifference between the two regions is increased, with the result beingthat the charge storing capacity of the photodiode is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above objects and other advantages of the present inventionwill become more apparent by describing in detail preferred embodimentsof the present invention with reference to the attached drawings inwhich:

[0027]FIG. 1 is a circuit diagram showing the constitution of a unitpixel of a conventional CMOS image sensor;

[0028]FIGS. 2A to 2C are sectional views showing the fabrication processfor a transfer transistor, a photodiode and a floating diffusion regionof the conventional image sensor;

[0029]FIGS. 3A to 3D are sectional views showing the fabrication processfor a transfer transistor, a photodiode and a floating diffusion regionof an image sensor according to the present invention;

[0030]FIG. 4A is a graphical illustration showing the simulation resultsof the potential changes in the conventional image sensor; and

[0031]FIG. 4B is a graphical illustration showing the simulation resultsof the potential changes in the image sensor according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] The preferred embodiment of the present invention will bedescribed referring to FIGS. 3A to 3D.

[0033] First, as shown in FIG. 3A, an ion implantation mask is preparedfor defining a photodiode-forming region on a p-type semiconductorsubstrate 30 on which a device isolating film 31 has been formed. Thenby using the mask, a first photoresist pattern PR1 is formed, and then,an ion implantation is carried out to form an n-type impurity region 32for a photodiode.

[0034] Then the first photoresist pattern PR1 is removed and, as shownin FIG. 3B, a second photoresist pattern PR2 is formed by using an ionimplantation mask for defining a transfer transistor region. Then adopant such as boron (B) or the like is ion-implanted to form a p-typeimpurity region 33.

[0035] Then as shown in FIG. 3C, a gate insulating film 34 and a gateelectrode 35 for the transfer transistor are formed in such a mannerthat a portion of the gate electrode 35 is overlapped with the p-typeimpurity region 33.

[0036] Then as shown in FIG. 3D, post processes for fabricating the CMOSimage sensor are carried out. That is, an insulating film spacer 36, aphotodiode p-type impurity region 37 and an n+floating diffusion region38 are formed.

[0037] Meanwhile, when a voltage is supplied to the gate electrode 35 ofthe transfer transistor, the voltage is adequately supplied to the ntype impurity region 32 of the photodiode, which is more deeply formedthan the n+ floating diffusion region 38. As a result, the read-outbecomes more advantageous.

[0038] That is, if the threshold voltage of the transfer transistor isincreased due to the p-type impurity region 33, then the transistorcannot serve as a native transistor and, therefore, the above measure isfor coping with the problems which may arise during a drop of theoperating voltage. However, if the transfer transistor is constitutedlike a pump circuit, the distance A between the p-type impurity region33 and the n-type impurity region 32 of the photodiode need not be takeninto account.

[0039] Meanwhile, the concentration of the n+ floating diffusion region38 is considerably higher than that of the p-type impurity region 33and, therefore, the operational characteristics are not much affected bythe p-type impurity region 33. Rather, the capacitance of the n+floating diffusion region 38 is decreased by the formation of the p-typeimpurity region 33 and, therefore, the conversion ratio increases, withthe result being that an improvement in sensitivity can be expected.

[0040] In the above described embodiment of the present invention, thedistance A between the p-type impurity region 33 and the photodiode ismade greater than the distance B between the p-type impurity region 33and the n+ floating diffusion region 38.

[0041]FIG. 4A is a graphical illustration showing the simulation resultsof the potential changes in the conventional image sensor which may becompared with the image sensor according to the present invention shownin FIG. 4B. In the potential distribution, the potential differencebetween the lines is 0.1 V.

[0042] In the case of the conventional image sensor shown in FIG. 4A,the potential difference between the photodiode PD and the floatingdiffusion region (FD) is 1.2 V, whereas in the image sensor of thepresent invention shown in FIG. 4B and having the p-type impurityregion, the potential difference between the photodiode and the floatingdiffusion region is increased to 1.8 V.

[0043] As to process variation, at the process tolerance of 0.1 μm forthe photodiode region, the conventional image sensor shows a maximumpotential barrier of 1.4 V and a minimum potential barrier of 0.9 V, thedifference between the two values being 0.5 V. In contrast to this, theimage sensor according to the present invention shows a maximumpotential barrier of 2.1 V, and a minimum potential barrier of 1.8 V,the difference between the two values being 0.3 V. Thus in the presentinvention, the maximum and minimum potential barrier values are markedlyincreased, while the difference between the two values is significantlydecreased.

[0044] Thus the characteristics of the photodiode according to thepresent invention are significantly improved compared with thoseobtained using the conventional self-aligning method. That is, if theself-aligning method is not adopted in forming the n-type impurityregion 32 without forming the p-type impurity region 33, then thevariation range of the potential barrier is 0.5 V at the processtolerance of 0.1 μ/m and, therefore, the saturation voltage differencesbetween different pixels become too high, with the result that theoverall sensitivity characteristics cannot be regulated.

[0045] In contrast to this, in the case where the p-type impurity region33 is formed as in the present invention, the difference between themaximum and minimum potential barriers is decreased to 0.3 V, andtherefore, the process variation is reduced. Accordingly, in the presentinvention, the punch-through voltage and the saturation voltage can beincreased, while maintaining a difference between potential barrierssimilar to that obtained through the conventional self-aligning method.

[0046] According to the present invention as described above, the n-typeimpurity region can be formed without resorting to the self-aligningmethod and, therefore, the process variation can be decreased during theion implantation. That is, when the ion implantation is carried out withan energy of 200 eV or more to form the n-type impurity region in theconventional technique, a channeling problem occurs. The presentinvention solves this channeling problem by forming the p-type impurityregion under the gate electrode of the transfer transistor and withinthe semiconductor substrate.

[0047] Further, the punch-through voltage can be reinforced between thephotodiode and the floating diffusion region even when the channellength of the transfer transistor is shortened. Further, the potentialbarrier between the photodiode and the floating diffusion region isheightened, thereby increasing the charge integrity of the photodiode.

[0048] As set forth above, the present invention is described based on aspecific preferred embodiment and the attached drawings, but it shouldbe apparent to those of ordinary skilled in the art that various changesand modifications can be made without departing from the spirit andscope of the present invention, which will be defined in the appendedclaims.

What is claimed is:
 1. A CMOS image sensor having a transfer transistorfor transferring charges from a photodiode to a floating diffusionregion, comprising: a semiconductor substrate; a gate electrode of thetransfer transistor, the gate electrode being formed on thesemiconductor substrate; a photodiode including a first conduction typefirst impurity region and a second conduction type second impurityregion, the first and second regions being formed at one end of the gateelectrode and within the semiconductor substrate; a floating diffusionregion including a second conduction type third impurity region, thethird region being formed at another end of the gate electrode andwithin the semiconductor substrate; and a first conduction type fourthimpurity region formed under the gate electrode and within thesemiconductor substrate, and isolated from the photodiode and thefloating diffusion region.
 2. The CMOS image sensor as claimed in claim1, wherein a distance between the fourth impurity region and thephotodiode is greater than a distance between the fourth impurity regionand the floating diffusion region.
 3. The CMOS image sensor as claimedin claim 1, wherein the first conduction type is a p-type, and thesecond conduction type is an n-type.
 4. A method for forming a CMOSimage sensor with a transfer transistor included therein fortransferring charges from a photodiode to a floating diffusion region,comprising steps of: forming a first conduction type first impurityregion in a photodiode-forming region and within a semiconductorsubstrate; forming a second conduction type second impurity region in atransfer transistor region and within the semiconductor substrate;forming a gate insulating film and a gate electrode of the transfertransistor on the semiconductor substrate, a portion of the gateelectrode being overlapped with the second impurity region; forming asecond conduction type third impurity region upon the first impurityregion and within the semiconductor substrate; and forming a firstconduction type floating diffusion region isolated from the photodioderegion and across the gate electrode.
 5. A method for forming a CMOSimage sensor with a transfer transistor included therein fortransferring charges from a photodiode to a floating diffusion region,comprising steps of: forming a first ion-implantation mask for defininga photodiode-forming region in a semiconductor substrate; forming afirst conduction type first impurity region within the semiconductorsubstrate by carrying out an ion-implantation; removing the firstion-implantation mask; forming a second ion-implantation mask fordefining a transfer transistor region on the semiconductor substrate;forming a second conduction type second impurity region within thesemiconductor substrate by carrying out an ion-implantation; removingthe second ion-implantation mask; forming a gate insulating film and agate electrode of the transfer transistor on the semiconductorsubstrate, a portion of the gate electrode being overlapped with thesecond impurity region; forming a second conduction type third impurityregion upon the first impurity region and within the semiconductorsubstrate; and forming a first conduction type floating diffusion regionwithin the semiconductor substrate, isolated from the photodiode regionand across the gate electrode.
 6. The method as claimed in claim 4,wherein a distance between the second impurity region and the photodiodeis greater than a distance between the second impurity region and thefloating diffusion region.
 7. The method as claimed in claim 6, whereinthe first conduction type is an n-type, and the second conduction typeis a p-type.
 8. The method as claimed in claim 6, wherein the secondimpurity region is formed by ion-implanting boron (B).
 9. The method asclaimed in claim 5, wherein a distance between the second impurityregion and the photodiode is greater than a distance between the secondimpurity region and the floating diffusion region.
 10. The method asclaimed in claim 9, wherein the first conduction type is an n-type, andthe second conduction type is a p-type.
 11. The method as claimed inclaim 9, wherein the second impurity region is formed by ion-implantingboron (B).